"""
Utility functions for decoding response bodies.
"""

import codecs
import collections
from io import BytesIO

import gzip
import zlib
import brotli
import zstandard as zstd

from typing import Union, Optional, AnyStr  # noqa


# We have a shared single-element cache for encoding and decoding.
# This is quite useful in practice, e.g.
# flow.request.content = flow.request.content.replace(b"foo", b"bar")
# does not require an .encode() call if content does not contain b"foo"
CachedDecode = collections.namedtuple(
    "CachedDecode", "encoded encoding errors decoded"
)
_cache = CachedDecode(None, None, None, None)


def decode(
    encoded: Optional[bytes], encoding: str, errors: str='strict'
) -> Optional[AnyStr]:
    """
    Decode the given input object

    Returns:
        The decoded value

    Raises:
        ValueError, if decoding fails.
    """
    if encoded is None:
        return None

    global _cache
    cached = (
        isinstance(encoded, bytes) and
        _cache.encoded == encoded and
        _cache.encoding == encoding and
        _cache.errors == errors
    )
    if cached:
        return _cache.decoded
    try:
        try:
            decoded = custom_decode[encoding](encoded)
        except KeyError:
            decoded = codecs.decode(encoded, encoding, errors)
        if encoding in ("gzip", "deflate", "br", "zstd"):
            _cache = CachedDecode(encoded, encoding, errors, decoded)
        return decoded
    except TypeError:
        raise
    except Exception as e:
        raise ValueError("{} when decoding {} with {}: {}".format(
            type(e).__name__,
            repr(encoded)[:10],
            repr(encoding),
            repr(e),
        ))


def encode(decoded: Optional[str], encoding: str, errors: str='strict') -> Optional[AnyStr]:
    """
    Encode the given input object

    Returns:
        The encoded value

    Raises:
        ValueError, if encoding fails.
    """
    if decoded is None:
        return None

    global _cache
    cached = (
        isinstance(decoded, bytes) and
        _cache.decoded == decoded and
        _cache.encoding == encoding and
        _cache.errors == errors
    )
    if cached:
        return _cache.encoded
    try:
        try:
            encoded = custom_encode[encoding](decoded)
        except KeyError:
            encoded = codecs.encode(decoded, encoding, errors)
        if encoding in ("gzip", "deflate", "br", "zstd"):
            _cache = CachedDecode(encoded, encoding, errors, decoded)
        return encoded
    except TypeError:
        raise
    except Exception as e:
        raise ValueError("{} when encoding {} with {}: {}".format(
            type(e).__name__,
            repr(decoded)[:10],
            repr(encoding),
            repr(e),
        ))


def identity(content):
    """
        Returns content unchanged. Identity is the default value of
        Accept-Encoding headers.
    """
    return content


def decode_gzip(content: bytes) -> bytes:
    if not content:
        return b""
    gfile = gzip.GzipFile(fileobj=BytesIO(content))
    return gfile.read()


def encode_gzip(content: bytes) -> bytes:
    s = BytesIO()
    gf = gzip.GzipFile(fileobj=s, mode='wb')
    gf.write(content)
    gf.close()
    return s.getvalue()


def decode_brotli(content: bytes) -> bytes:
    if not content:
        return b""
    return brotli.decompress(content)


def encode_brotli(content: bytes) -> bytes:
    return brotli.compress(content)


def decode_zstd(content: bytes) -> bytes:
    if not content:
        return b""
    zstd_ctx = zstd.ZstdDecompressor()
    try:
        return zstd_ctx.decompress(content)
    except zstd.ZstdError:
        # If the zstd stream is streamed without a size header,
        # try decoding with a 10MiB output buffer
        return zstd_ctx.decompress(content, max_output_size=10 * 2**20)


def encode_zstd(content: bytes) -> bytes:
    zstd_ctx = zstd.ZstdCompressor()
    return zstd_ctx.compress(content)


def decode_deflate(content: bytes) -> bytes:
    """
        Returns decompressed data for DEFLATE. Some servers may respond with
        compressed data without a zlib header or checksum. An undocumented
        feature of zlib permits the lenient decompression of data missing both
        values.

        http://bugs.python.org/issue5784
    """
    if not content:
        return b""
    try:
        return zlib.decompress(content)
    except zlib.error:
        return zlib.decompress(content, -15)


def encode_deflate(content: bytes) -> bytes:
    """
        Returns compressed content, always including zlib header and checksum.
    """
    return zlib.compress(content)


custom_decode = {
    "none": identity,
    "identity": identity,
    "gzip": decode_gzip,
    "deflate": decode_deflate,
    "br": decode_brotli,
    "zstd": decode_zstd,
}
custom_encode = {
    "none": identity,
    "identity": identity,
    "gzip": encode_gzip,
    "deflate": encode_deflate,
    "br": encode_brotli,
    "zstd": encode_zstd,
}

__all__ = ["encode", "decode"]
